This invention relates to a signal generating and control circuit for a telephone system such as a private branch exchange (PBX), which controls operation of audible and visual subscriber station instrument signaling devices.
In such systems a composite signal comprising audible and visual control signals alternately spaced in time, selectively energizes the ringer or message waiting lamp in subscriber instruments. These audible and visual signals energize ringers and neon lamps respectively at subscriber stations of a PBX via a common bus, to which each station may be selectively connected, via an individual single wire path for each station. The present invention can be utilized with the circuit disclosed in U.S. Pat. No. 4,266,101, which is hereby incorporated by reference.
A common problem with existing systems is "bell tap", which is transient energizing of the ringer, which occurs when switching between the audible signal and the visual signal states. In addition, a rapid load change, which can occur when a hand set of a subscriber station receiving a visual signal is taken off-hook, may cause "bell tap" to occur at other subscriber stations connected to the same signal source.